This invention relates to an improved and electronically controlled fuel injection system having an oxygen content detector mounted in the exhaust system thereof to control the fuel injection in accordance with the oxygen content in the exhaust system, thereby reducing the harmful components in the exhaust gas.
Generally the amount of oxygen in the exhaust gas varies with air-fuel ratio as shown in FIG. 1. Conventional oxygen content detectors generate in response thereto an electromotive force as shown in FIG. 2 having a peculiar point at the air-fuel ratio of about 14.8 which corresponds to the starting point of oxygen (O.sub.2) content in the curve of FIG. 1. Conventional oxygen content detectors are formed of a metal oxide, such as Zirconium dioxide and Titanium dioxide, and generate an electromotive force when heated. A difference of oxygen content is produced between one side of the detector and the other by the movement of oxygen ions.
In a conventional fuel injection system mounting such oxygen content detector, the air-fuel ratio is controlled at no other value than the theoretical one of 14.8, since the electromotive force of the detector changes abruptly at the point of about 14.8 as seen from FIG. 2, making detection of the point easy. The detector supplies the control signal generated therein to a computer, which operates to modify the injection pulse so that the air-fuel ratio always keeps a value of about 14.8 by increasing the fuel when the ratio is above 14.8 and decreasing it when below 14.8.